1. Field of the Invention
This invention relates to a display terminal for a character-image information system such as Videotex.
2. Description of the Prior Art
There have been proposed character-image information systems such as CAPTAIN (character and pattern telephone access information network) system and NAPLPS (North American Presentation Level Protocol Syntax).
FIG. 1 schematically shows an example of such character-image information system. In FIG. 1, reference numeral 10 designates a data center for transmitting a character-image information to a user terminal 30. The data center 10 and the user terminal 30 are coupled with each other through a telephone cable 1, for example, so that the bidirectional data communication can be carried out therebetween.
The user terminal 30 is provided with a microcomputer and this microcomputer comprises a CPU (central processing unit) 31, a ROM (read only memory) 32 which stores a program for controlling the functions of the system such as reception, display, print and transmission or the like and a RAM (random access memory) 33.
Reference numeral 21 designates a MODEM (modulator and demodulator) and this MODEM 21 demodulates a modulated data transmitted from the data center 10 to the user terminal 30 or modulates a signal transmitted from the user terminal 30 to the data center 10. The MODEM 21 is connected through an I/O port 34 to a system bus 35.
Reference numeral 22 designates a keyboard and 23 a printer for making a hard copy of image information. They are connected to the system bus 35 through I/O ports 36 and 37, respectively.
Reference numeral 24 designates a video RAM which stores display data consisting of one page amount (one picture frame amount), 25 a memory controller for forming an address signal and read and write signals of the video RAM 24 and, 26 a CRT (cathode ray tube) or display controller which controls a CRT or monitor 27 to display an image information. They are connected one another and to the system bus 35, respectively,
When a desired image information is selected by the keyboard 22, the CPU 31 accepts it so that information request signal is supplied through the I/O port 34 to the MODEM 21 in which it is modulated and thus modulated information request signal is supplied through the telephone cable 1 to the data center 10.
When the data center 10 is supplied with such request signal, the data center 10 modulates the requested image information and transmits the modulated image information to the user terminal 30. In the user terminal 30, the image information is demodulated by the MODEM 21, stored through the I/O port 34 to the RAM 33 and then written in the video RAM 24 in turn.
The memory controller 25 forms a read address signal and a read signal which are in synchronsim with vertical and horizontal scannings. These signals are respectively supplied to the video RAM 24 in a time-division manner with a write signal so that the display data is sequentially read out from the video RAM 24 in synchronism with the vertical and horizontal scannings. The display data read out from the video RAM 24 is decoded to three primary color signals R, G and B by the video controller 26 and they are fed to the monitor 27. Thus the color picture image transmitted from the data center 10 is displayed on the display screen of the monitor 27. When a hard copy request key (not shown) of the keyborad 22 is depressed, the display data is supplied through the I/O port 37 to the printer 23 in which it is printed out on a printing paper. Thus the hard copy of the display data can be made as required.
For example, the CAPTAIN system uses as its format a noraml mode and a multi-image mode to transmit an image data. Further, the multi-image mode includes a center-oriented multi-image mode and a terminal-oriented multi-image mode. In this case, in the terminal-oriented multi-image mode, the user can watch the plurality of images of normal mode as multi-images simultaneously. More particularly, in the picture frame of a screen of the monitor 27, one page is formed by 496 dots x 408 dots as shown in FIG. 2, while in the normal mode, 2 dots each in the horizontal and vertical directions, or 4 dots in total are expressed by the same information as shown by cross-hatched circles in FIG. 2 so that the picture frame of the normal mode becomes equal to the picture frame in which one page is formed of 248 dots.times.204 dots.
In the multi-image mode, one page is formed of 496 dots.times.408 dots and one picture frame of the screen is divided into 4 equal parts and then 4 small picture frames #1 to #4 can be displayed as shown in FIG. 3. Also the memory area of the video RAM 24 is divided into 4 equal areas A.sub.1 to A.sub.4 corresponding to the 4 small picture frames #1 to #4 as shown in FIG. 4.
In the terminal-oriented multi-image mode, the image information or data of each picture frame transmitted from the data center 10 are stored in the areas A.sub.1 to A.sub.4 of the video RAM 24. When picture frames are sequentially transmitted from the data center 10, the image information is sequentially writted in the areas A.sub.1 to A.sub.4 of the video RAM 24 in the order of A.sub.1 .fwdarw.A.sub.2 .fwdarw.A.sub.3 .fwdarw.A.sub.4 .fwdarw.A.sub.1 .fwdarw.A.sub.2 . . . .
Accordingly, on the screen of the monitor 27, each of the 4 small picture frames #1 to #4 displays different display information and is changing its display content as a signal is transmitted from the data center 10.
By the way, in the terminal-oriented multi-image mode, there is the possibility that the user may want to watch any one of the 4 small picture frames #1 to #4 in an unchanged or fixed state. However, such user's request can not be satisfied. On the contrary, the content or data stored in the video RAM 24 is sequentially rewritten by the information transmitted from the data center 10 and thence the desired small picture frame of small picture frames in multi-image mode can not be viewed in an unchanged state.